1. Field of the Invention
The present invention relates generally to the continuous manufacture of flat glass in ribbon form by the float process, and more particularly to devices for cooling and shielding the exit end of a float tank from heat radiating from surrounding bodies at higher temperatures.
2. Description of the Prior Art
A conventional form of a float glass forming apparatus is illustrated and described in U.S. Pat. No. 3,607,202 issued on Sept. 9, 1971. As therein explained, the manufacture of flat glass by the float process involves delivering molten glass at a controlled rate onto a relatively wide bath of molten metal such as tin or an alloy of tin, and advancing it along the surface of the bath under physical and thermal conditions which assure that (1) a layer of molten glass will be established on the metal bath, (2) the glass in the layer can flow laterally to develop on the surface of the bath a buoyant body of molten glass of stable thickness, and (3) that the buoyant body in ribbon form will be continuously advanced along the bath and sufficiently cooled as it advances to permit it to be taken from the bath by lift-out conveying rolls. Above the float bath of molten metal a superstructure forming a tightly enclosed head-space or plenum chamber is provided to contain a so-called float atmosphere.
Such a float glass forming apparatus generally includes an enclosed, roofed refractory structure surrounded by a metallic casing, with the molten metal being contained within the lower section of the refractory structure. Radiant heaters and coolers conventionally provided within the headspace maintain the ribbon at various elevated temperatures along its length according to a prescribed pattern as required to practice the above-described process. The temperature of the glass in the buoyant layer or ribbon must be carefully controlled so as to progressively cool it from the entrance end to the discharge end of the chamber, by which time the ribbon surfaces reach a temperature of about 1200.degree. F. (643.degree. C.) at which the ribbon is sufficiently stiffened to allow its transfer to a lehr on mechanical conveying means.
A cooler such as disclosed in U.S. Pat. No. 3,433,614, issued on Mar. 19, 1969, is attached to the metallic casing enclosing the refractory structure of the lower section at its exit end for preventing warping of the casing and consequent separation from the refractory structure due to the differential expansion of the different materials under the severe temperature encountered in this area. In order to permit the glass ribbon to be removed from the surface of the molten metal along a path which is as nearly horizontal as possible, that is, without being subjected to a substantial change in direction at this critical stage, it is desirable to maintain the level of the molten metal very near the top of the end wall of the refractory structure. The molten metal flows over, or through the joints in, the refractory blocks to contact in its molten state the metallic cooler. When the molten metal contacts the exposed surfaces of the cooler, it corrosively attacks and subsequently eats through the cooler, thus allowing the molten metal to escape. The cooler itself is unable to solidify the molten metal coming into contact therewith because of the aforementioned severe temperature conditions due to heat radiating from the hot glass ribbon and other surrounding bodies such as the glass lift-out conveying rolls.
It has been proposed in the past to shield the exposed surfaces of the atmosphere cooler from its surrounding hotter environment. However, because of the small clearance between the bottom surface of the glass ribbon and the top surface of the cooler, it is not practical to place conventional heat insulating material such as millboard or hotboard insulation therebetween since the glass ribbon in its plastic form may sag and contact the insulation material. Such contact marks and renders the glass unfit for commercial use. Another disadvantage of conventional insulating materials is that they are not sufficiently rigid to take a permanent set when they are formed into a desired shape so as to remain in place over the cooler.